Fix whitespace padding width with tabs and Unicode combining characters

Tabs are replaced by tabs to preserve the string width,
all other characters are replaced by a single space.
Moreover we now count the unicode graphemes instead of UTF16 units.
It still has the problem that some characters (Emoji, CJK) are wider even
in monospace fonts, but that's not that easy to determine.

src/Framework/Framework/Compilation/ViewCompiler/DefaultViewCompiler.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Globalization;
 using System.Linq;
 using DotVVM.Framework.Compilation.ControlTree;
 using DotVVM.Framework.Compilation.ControlTree.Resolved;
@@ -148,10 +149,19 @@ public override string ToString()
                     ""
                 );
                 string error;
-                if (ContextLine is {})
+                if (ContextLine is {} contextLine)
                 {
-                    var errorHighlight = new string(' ', CharPosition ?? 1) + new string('^', HighlightLength);
-                    error = $"{fileLocation}: Dotvvm Compilation Warning\n{ContextLine}\n{errorHighlight} {Message}";
+                    var graphemeIndices = StringInfo.ParseCombiningCharacters(contextLine.Substring(0, Math.Min(contextLine.Length, CharPosition ?? 0)));
+                    var padding = string.Concat(
+                        graphemeIndices.Select(
+                            startIndex => contextLine[startIndex] switch {
+                                '\t' => "\t",
+                                _ => " "
+                            }
+                        )
+                    );
+                    var errorHighlight = padding + new string('^', HighlightLength);
+                    error = $"{fileLocation}: Dotvvm Compilation Warning\n{contextLine}\n{errorHighlight} {Message}";
                 }
                 else
                 {